A standard drum centrifuge has a housing in which a foraminous drum is rotated at high speed about its axis. A suspension is fed to the interior of the drum so that it is thrown centrifugally against the wall thereof. At first the suspension forms an annular body in the drum having an inner surface centered on the axis, then the body stratifies and the liquid phase passes through the drum and the solid phase stays behind on the inner surface of the drum as a filter cake. This inner layer of liquid passes radially outward through the layer of solids until same is substantially dry. As a rule the drum is filled and refilled several times until the filter cake builds up to a desired depth. Then this cake is washed by passing a liquid through it, and then it is centrifuged to an extremely low moisture content. Subsequently a hot gas can be passed through it to further dry it, and finally it is physically stripped out of the drum, same is regenerated, and the cycle is restarted.
The level, that is the radial position relative to the drum rotation axis, of the inner surface of the annular body formed by the liquid and solid fractions can be sensed by a detector such as described in German patent document 3,726,227 filed 07 Aug. 1989 by peter Sedlmayer, or by a system such as described in patent application 07/614,808 filed 16 Nov. 1990 by Rainer Kampschulte. Such sensors can even detect when the liquid has run through the cake and the top of the body in the centrifuge is in fact formed by solids, the so-called dry point.
The centrifuging and drying time is fairly long compared to the time necessary to spin the liquid fraction out of each batch. Thus for maximum efficiency each batch must be as large as possible, capable of filling the drum inward to a level just below the inlet. On the other hand the filter cake must be reduced to a fairly low residual moisture content.
As discussed in East German patent 218,283 (D. Trumper), and in West German patents 1,036,763 (J. Hertrich), 1,186,411 (K. Zeppenfeld et al), 2,441,849 (H. Bitus), 2,525,232 (W. Schillig), 2,649,037 (H.Papezik), and 3,615,013 (P. Franzen) the rate at which the liquid level, distinguished from the underlying solids level, drops in the drum is a function of the composition of the fractions. The particle size of the solid fraction, viscosity of the liquid fraction, thickness of the filter cake, thickness of the base layer underlying the cake, and other factors all affect the rate at which liquid can be driven out of the suspension being filtered. Existing technology does not allow these factors to be taken into account, so the refilling time and cycling time are usually set somewhat longer than is strictly necessary to produce a filter cake of the desired low moisture content, since to err on the side of a too wet product is to produce something that will have to be recycled through the drum centrifuge before it can be used.